1. Field of the Invention
The present invention relates to an objective for compact lens-shutter cameras whose diaphragm is arranged behind the whole optical system, and more particularly to a compact wide angle objective with a simple construction for cameras for a spread use.
2. Description of the Prior Art
With a compact lens-shutter camera for cameras for a spread use, a wide angle objective with a diaphragm arranged behind the whole lens system is advantageous for accommodation in the camera, for compactization of the camera or the objective itself and for realization of precise focusing. As this kind of objective, triplet type objectives with F No. 4.about.2.8 and a total angular field coverage of 60.degree..about.64.degree. have been proposed. With a wide angle triplet type objective, since a astigmatic difference tends to become large at a marginal angular field, it was conventional to construct this type of objective by using lens elements formed of glass with a high refractive index. For example, in Japanese patent application laid-open number Sho. 59-34510, all lens elements, one or two of which have an aspherical surface, are formed of glass with a high refractive index to construct an objective with a total angular field coverage of 64.degree., and F No. 3.5. In Japanese patent application laid-open number Sho. 57-2012, an objective with F No. 4 and a total angular field coverage of 64.degree. has been realized by using glass with a high refractive index. But forming a lens element of glass of a high refractive index incurs an increase in the manufacturing cost.
On the other hand, in U.S. Pat. No. 3,784,287 or in Japanese patent application laid-open number Sho. 61-272710, a wide angle triplet type objective without using glass of a high refractive index have been proposed, but the objective in the former prior art is of F No. 5.6 and the objective in the latter prior art is of F No. 8.0 which are relatively small.
Further, in Japanese patent application laid-open number Sho. 62-183420, a wide angle triplet type objective with a diaphragm arranged behind the lens system comprising from the object side, a first positive lens formed of glass with a high refractive index, a second negative lens formed of plastic, a third positive lens formed of plastic. In the objective, the second and the third lenses are formed of plastic, so that manufacturing cost is low as well as the change in the back focus depending on the temperature is cancelled. And the first lens is formed of glass with a high refractive index, so that aberrations are corrected well. However, for securing sufficient marginal light rays, it is necessary to construct a diameter of the first lens so large that manufacturing cost tends to become high. Because, glass material, which is much expensive than plastic, is employed of the largest volume lens in the whole lens system.